Hi
have had that problem on the raincoast here (N of Vancouver)  best thing I
have found is Starbrite mold and mildew cleaner.  This is a marine product,
like very strong bleach., spray it on and watch the mold disappear.  then
rinse with water.  I have done this with the top still on the van but have
to be careful not to discolour tent cnavas or beds.-----

rgds

Rick